In the past, as an exhaust gas analyzing system based on a dilution sampling method that dilutes exhaust gas emitted from an internal combustion engine, there has been a system provided with: a main flow path through which exhaust gas emitted from an exhaust pipe of an internal combustion engine flows; a diluent gas flow path that meets the main flow path, through which diluent gas for diluting the exhaust gas flows; and a flow rate control part that is provided on a downstream side of a meeting point between the main flow path and the diluent gas flow path to control a flow rate of diluted exhaust gas produced by diluting the exhaust gas with the diluent gas.
In the dilution sampling method, as one of meters for measuring a flow rate of the exhaust gas emitted from an exhaust pipe, an exhaust gas flowmeter that performs measurement by a tracer method is known.
The exhaust gas flowmeter is, for example, as disclosed in JP-A-62-5151, provided with: a sampling line for, from a main flow path on an upstream side of a meeting point, sampling raw exhaust gas that is exhaust gas before dilution; a sampling line for, from the main flow path on a downstream side of the meeting point, sampling diluted exhaust gas that is the exhaust gas after the dilution; concentration meters that are connected to the respective sampling lines to measure concentrations of target components (such as CO2) contained in the sampled gases; and an arithmetic unit that calculates a flow rate of the raw exhaust gas from the target component concentrations measured by the respective concentration meters, and a diluted exhaust gas flow rate controlled by a flow rate control part.
According to the exhaust gas flowmeter configured as described, the respective concentration meters instantaneously measure the concentrations of the target components contained in the raw exhaust gas and the diluted exhaust gas, and thereby the flow rate of the raw exhaust gas can be instantaneously measured on the assumption that a ratio between the concentrations is equal to a ratio of the dilution by diluent gas.
Meanwhile, exhaust gas emitted from an internal combustion engine contains large amounts of components acting as interference components such as moisture. Methods for removing the moisture include a method that provides a drain separator between a sampling line and a concentration meter to remove the moisture contained in the exhaust gas, and then introduces the exhaust gas into the concentration meter (so-called Dry measurement). In this case, the concentration of a target component is increased as compared with that in a state where the exhaust gas contains the moisture, and therefore on the basis of a conversion expression (Dry to Wet conversion expression) provided in the regulation (40 CFR Part 1065), the concentration is converted to the concentration of the target component contained in the exhaust gas immediately after the emission from the internal combustion engine.
However, in practice, during a period when the exhaust gas emitted from the internal combustion engine flows through an exhaust pipe, the moisture contained in the exhaust gas may be condensed or adsorbed in the exhaust pipe. In this case, between the exhaust gas immediately after the emission from the internal combustion engine and the exhaust gas (the above-described raw exhaust gas) emitted from the exhaust pipe, a difference in contained moisture amount occurs, and the concentration of the target component as a result of the conversion by the conversion expression does not indicate the concentration of the target component contained in the raw exhaust gas. Also, it is difficult to know how much the condensation, adsorption, or the like occurs in the exhaust pipe, and therefore it is difficult to accurately measure the concentration of the target component contained in the raw exhaust gas.
On the other hand, a method that, without using a drain separator, measures the concentration of the target component in a state where the exhaust gas emitted from the exhaust pipe contains the moisture (so-called Wet measurement) is also possible; however, in this method, in the sampling line or concentration meter through which the raw exhaust gas particularly containing a large amount of moisture flows, the moisture contained in the raw exhaust gas is condensed or adsorbed. In the case where the target component is a component having solubility, such as CO2, this may cause a measurement error because the target component is dissolved in the condensed or adsorbed moisture, and even in this case, there occurs a problem of being unable to accurately measure the concentration of the target component.